There are 2 types of servers. SIP which handles the actual registration and call initiation / termination signaling and RTP server which streams the audio to you.

RTP server is chosen based upon the location of the Vonage terminating GW your call goes through and on an outbound call, depending upon what number is dialed will determine it's routing. So each and every outbound you make depending upon number dialed could stream audio to you from 1 of 4 different geographical locations. On inbound calls, depending upon what market your number comes from, will determine which Inbound location streams your Inbound audio.

SIP servers are based upon a group of 4 and that is just determined randomly. You will register to 1 of the 4 which are all in 4 different locations for redundancy and over a time period could be registered to any one of them.

DNS servers will look up any hostnames for any service you need to resolve to. The term Open DNS servers just mean they are DNS servers that are not assigned to you by your ISP but still process DNS requests from clients outside of their own domain or network. Some DNS servers will only process requests from clients that are within their network and therefore are not considered Open DNS servers.

But the DNS servers you put in a router or on your PC do not change that way a Vonage adapter resolves Vonage based server hostnames for SIP and TFTP requests due to it uses the Vonage DNS servers which are provisioned into it's profile